LK Heteromorph Ammonite Mariella
Mariella (Plesioturrilites) sp.
Main Street Limestone
Lower Cretaceous (Albian)
Tarrant County, Texas
This is a segment of a large heteromorphy ammonite. The specimen comes with its original matrix but the fossil is not attached to the rock. The specimen has two whorls.

The genus is a medium to large conical shell that can reach one foot in length with a diameter of 8 inches. The body chamber uncoils but is rarely found in place. The whorls coil tightly with an overlap that obscures one row of tubercles. Two sets(four rows) of tubercles are found on each whorl and the two sets are separated by a spiral band. Tubercles are lengthened and aligned obliquely on the whorl flank. The suture pattern is rarely preserved.

The Main Street Limestone totals about 50 feet in thickness at Fort Worth. The formation with little variations from top to bottom, is composed of regularly alternating strata of straw-colored marl and chalky, or hard fairly pure limestone. The bands do not usually exceed one foot in thickness and the formation has a deceptive similarity to the Fort Worth limestone.